This invention relates to a magneto-optic thin film for memories composed of four or five elements.
Among the conventional magneto-optic thin films for memories, a Mn-Bi film is most prominent in readout efficiency. The film performs a memory function in the following manner. First of all, a laser beam is irradiated to a micro region of the film magnetized in advance in a direction perpendicular to the surface of the film. The micro region receiving the laser beam absorbs the light energy of the beam to raise the temperature of the micro region. When the temperature exceeds a Curie point, the micro region is demagnetized. Then, the laser beam irradiation is stopped. This causes the temperature of the micro region to lower below the Curie point, resulting in magnetization in the opposite direction to that of the other resions of the film. This process is called a writing by Curie point method.
The performance of a magneto-optic thin film for memories is evaluated from the points of its writing power and its readout signal-to-noise ratio. In the case of Curie point method, the magnitude of writing power is proportional to the difference between a room temperature and a Curie point of the film. It follows that the writing power of a magneto-optic thin film having a low Curie point is small. A small writing power is an absolute requisite for a magneto-optic thin film. On the other hand, a magneto-optic figure of merit 2F/.alpha., "F" denoting a Faraday rotation angle for a unit thickness of the film and ".alpha." representing a light absorption coefficient, constitutes a parameter for evaluating a readout signal-to-noise ratio of the film. A large value of 2F/.alpha. is also absolutely required for a magneto-optic thin film.
J. Appl. Phys. Vol. 43, p. 2875 (1972) discloses a magneto-optic thin film of Mn-Bi-Ti system. Further, Solid State Communication Vol. 14, p. 33 (1974) teaches systems of Mn.sub.5 Ni.sub.2 Bi.sub.4 and Mn.sub.3 Cu.sub.4 Bi.sub.4. These substances have relatively low Curie points.